Frames for decorative art, mirrors, posters and like are conventionally formed from a plurality (usually four) of sides, or frame parts, which are cut from longer pieces of mouldings, of up to about 16 feet in length, and thereafter joined to each other at mitered ends. Small, custom framers literally have thousands of such decorated frame parts, either in their own inventory, or readily available from a number of suppliers. However, when their customers have 3-dimensional objects to be displayed, even though the customer likes a particular decorated frame, they are often disappointed to find that specialized framing is necessary to accommodate the thickness of the article to be displayed. Not only are there increases in the expense and labor required to assemble the construction, but in the present state of the art, none of the many beautifully decorated frames carried by, or available to, small, custom framers can be used. In addition, obviously, in making individual shadowboxes much effort is needed to carefully measure and miter the individual frame members and to secure proper fitting. It would be desirable, therefore to provide shadow box base mouldings based on a novel eight-sided cross section which can be mitered and assembled with a minimum of effort into a shadowboxes decorated with the broadest selection of ordinary picture frames and such mouldings in lengths of up to about 16 feet are provided in accordance with the instant invention.